Morphogen Delivery by Osteoconductive Nanoparticles Instructs Stromal Cell Spheroid Phenotype

Tissue engineering approaches for dental pulp regeneration: The development of novel bioactive materials using pharmacological epigenetic inhibitors

The drive for minimally invasive endodontic treatment strategies has shifted focus from technically complex and destructive root canal treatments towards more conservative vital pulp treatment. However, novel approaches to maintaining dental pulp vitality after disease or trauma will require the development of innovative, biologically-driven regenerative medicine strategies. For example, cell-homing and cell-based therapies have recently been developed in vitro and trialled in preclinical models to study dental pulp regeneration. These approaches utilise natural and synthetic scaffolds that can deliver a range of bioactive pharmacological epigenetic modulators (HDACis, DNMTis, and ncRNAs), which are cost-effective and easily applied to stimulate pulp tissue regrowth. Unfortunately, many biological factors hinder the clinical development of regenerative therapies, including a lack of blood supply and poor infection control in the necrotic root canal system. Additional challenges include a need for clinically relevant models and manufacturing challenges such as scalability, cost concerns, and regulatory issues. This review will describe the current state of bioactive-biomaterial/scaffold-based engineering strategies to stimulate dentine-pulp regeneration, explicitly focusing on epigenetic modulators and therapeutic pharmacological inhibition. It will highlight the components of dental pulp regenerative approaches, describe their current limitations, and offer suggestions for the effective translation of novel epigenetic-laden bioactive materials for innovative therapeutics.

Injectable MSC Spheroid and Microgel Granular Composites for Engineering Tissue

Many cell types require direct cell-cell interactions for differentiation and function; yet, this can be challenging to incorporate into 3-dimensional (3D) structures for the engineering of tissues. Here, a new approach is introduced that combines aggregates of cells (spheroids) with similarly-sized hydrogel particles (microgels) to form granular composites that are injectable, undergo interparticle crosslinking via light for initial stabilization, permit cell-cell contacts for cell signaling, and allow spheroid fusion and growth. One area where this is important is in cartilage tissue engineering, as cell-cell contacts are crucial to chondrogenesis and are missing in many tissue engineering approaches. To address this, granular composites are developed from adult porcine mesenchymal stromal cell (MSC) spheroids and hyaluronic acid microgels and simulations and experimental analyses are used to establish the importance of initial MSC spheroid to microgel volume ratios to balance mechanical support with tissue growth. Long-term chondrogenic cultures of granular composites produce engineered cartilage tissue with extensive matrix deposition and mechanical properties within the range of cartilage, as well as integration with native tissue. Altogether, a new strategy of injectable granular composites is developed that leverages the benefits of cell-cell interactions through spheroids with the mechanical stabilization afforded with engineered hydrogels.

Osteobiologies for Spinal Fusion from Biological Mechanisms to Clinical Applications: A Narrative Review

Degenerative lumbar spinal disease (DLSD), including spondylolisthesis and spinal stenosis, is increasing due to the aging population. Along with the disease severity, lumbar interbody fusion (LIF) is a mainstay of surgical treatment through decompression, the restoration of intervertebral heights, and the stabilization of motion segments. Currently, pseudoarthrosis after LIF is an important and unsolved issue, which is closely related to osteobiologies. Of the many signaling pathways, the bone morphogenetic protein (BMP) signaling pathway contributes to osteoblast differentiation, which is generally regulated by SMAD proteins as common in the TGF-β superfamily. BMP-2 and -4 are also inter-connected with Wnt/β-catenin, Notch, and FGF signaling pathways. With the potent potential for osteoinduction in BMP-2 and -4, the combination of allogenous bone and recombinant human BMPs (rhBMPs) is currently an ideal fusion material, which has equalized or improved fusion rates compared to traditional materials. However, safety issues in the dosage of BMP remain, so overcoming current limitations will provide significant advancement in spine surgery. In the future, translational research and the application of clinical study will be important to overcome the current limitations of spinal surgery.

Strategies to capitalize on cell spheroid therapeutic potential for tissue repair and disease modeling

Cell therapies offer a tailorable, personalized treatment for use in tissue engineering to address defects arising from trauma, inefficient wound repair, or congenital malformation. However, most cell therapies have achieved limited success to date. Typically injected in solution as monodispersed cells, transplanted cells exhibit rapid cell death or insufficient retention at the site, thereby limiting their intended effects to only a few days. Spheroids, which are dense, three-dimensional (3D) aggregates of cells, enhance the beneficial effects of cell therapies by increasing and prolonging cell-cell and cell-matrix signaling. The use of spheroids is currently under investigation for many cell types. Among cells under evaluation, spheroids formed of mesenchymal stromal cells (MSCs) are particularly promising. MSC spheroids not only exhibit increased cell survival and retained differentiation, but they also secrete a potent secretome that promotes angiogenesis, reduces inflammation, and attracts endogenous host cells to promote tissue regeneration and repair. However, the clinical translation of spheroids has lagged behind promising preclinical outcomes due to hurdles in their formation, instruction, and use that have yet to be overcome. This review will describe the current state of preclinical spheroid research and highlight two key examples of spheroid use in clinically relevant disease modeling. It will highlight techniques used to instruct the phenotype and function of spheroids, describe current limitations to their use, and offer suggestions for the effective translation of cell spheroids for therapeutic treatments.

Sulfated Alginate Hydrogels Prolong the Therapeutic Potential of MSC Spheroids by Sequestering the Secretome

Cell-based approaches to tissue repair suffer from rapid cell death upon implantation, limiting the window for therapeutic intervention. Despite robust lineage-specific differentiation potential in vitro, the function of transplanted mesenchymal stromal cells (MSCs) in vivo is largely attributed to their potent secretome comprising a variety of growth factors (GFs). Furthermore, GF secretion is markedly increased when MSCs are formed into spheroids. Native GFs are sequestered within the extracellular matrix (ECM) via sulfated glycosaminoglycans, increasing the potency of GF signaling compared to their unbound form. To address the critical need to prolong the efficacy of transplanted cells, alginate hydrogels are modified with sulfate groups to sequester endogenous heparin-binding GFs secreted by MSC spheroids. The influence of crosslinking method and alginate modification is assessed on mechanical properties, degradation rate, and degree of sulfate modification. Sulfated alginate hydrogels sequester a mixture of MSC-secreted endogenous biomolecules, thereby prolonging the therapeutic effect of MSC spheroids for tissue regeneration. GFs are sequestered for longer durations within sulfated hydrogels and retain their bioactivity to regulate endothelial cell tubulogenesis and myoblast infiltration. This platform has the potential to prolong the therapeutic benefit of the MSC secretome and serve as a valuable tool for investigating GF sequestration.

Biomaterials-assisted spheroid engineering for regenerative therapy

Cell-based therapy is a promising approach in the field of regenerative medicine. As cells are formed into spheroids, their survival, functions, and engraftment in the transplanted site are significantly improved compared to single cell transplantation. To improve the therapeutic effect of cell spheroids even further, various biomaterials (e.g., nano- or microparticles, fibers, and hydrogels) have been developed for spheroid engineering. These biomaterials not only can control the overall spheroid formation (e.g., size, shape, aggregation speed, and degree of compaction), but also can regulate cell-to-cell and cell-to-matrix interactions in spheroids. Therefore, cell spheroids in synergy with biomaterials have recently emerged for cell-based regenerative therapy. Biomaterials-assisted spheroid engineering has been extensively studied for regeneration of bone or/and cartilage defects, critical limb ischemia, and myocardial infarction. Furthermore, it has been expanded to pancreas islets and hair follicle transplantation. This paper comprehensively reviews biomaterials-assisted spheroid engineering for regenerative therapy.

Growth factor delivery using extracellular matrix-mimicking substrates for musculoskeletal tissue engineering and repair

Therapeutic approaches for musculoskeletal tissue regeneration commonly employ growth factors (GFs) to influence neighboring cells and promote migration, proliferation, or differentiation. Despite promising results in preclinical models, the use of inductive biomacromolecules has achieved limited success in translation to the clinic. The field has yet to sufficiently overcome substantial hurdles such as poor spatiotemporal control and supraphysiological dosages, which commonly result in detrimental side effects. Physiological presentation and retention of biomacromolecules is regulated by the extracellular matrix (ECM), which acts as a reservoir for GFs via electrostatic interactions. Advances in the manipulation of extracellular proteins, decellularized tissues, and synthetic ECM-mimetic applications across a range of biomaterials have increased the ability to direct the presentation of GFs. Successful application of biomaterial technologies utilizing ECM mimetics increases tissue regeneration without the reliance on supraphysiological doses of inductive biomacromolecules. This review describes recent strategies to manage GF presentation using ECM-mimetic substrates for the regeneration of bone, cartilage, and muscle.

Biomaterials-assisted spheroid engineering for regenerative therapy

Cell-based therapy is a promising approach in the field of regenerative medicine. As cells are formed into spheroids, their survival, functions, and engraftment in the transplanted site are significantly improved compared to single cell transplantation. To improve the therapeutic effect of cell spheroids even further, various biomaterials (e.g., nano- or microparticles, fibers, and hydrogels) have been developed for spheroid engineering. These biomaterials not only can control the overall spheroid formation (e.g., size, shape, aggregation speed, and degree of compaction), but also can regulate cell-to-cell and cell-to-matrix interactions in spheroids. Therefore, cell spheroids in synergy with biomaterials have recently emerged for cell-based regenerative therapy. Biomaterials-assisted spheroid engineering has been extensively studied for regeneration of bone or/and cartilage defects, critical limb ischemia, and myocardial infarction. Furthermore, it has been expanded to pancreas islets and hair follicle transplantation. This paper comprehensively reviews biomaterials-assisted spheroid engineering for regenerative therapy. [BMB Reports 2021; 54(7): 356-367].

Hydrogel mechanics are a key driver of bone formation by mesenchymal stromal cell spheroids

Mesenchymal stromal cells (MSCs) can promote tissue repair in regenerative medicine, and their therapeutic potential is further enhanced via spheroid formation. Stress relaxation of hydrogels has emerged as a potent stimulus to enhance MSC spreading and osteogenic differentiation, but the effect of hydrogel viscoelasticity on MSC spheroids has not been reported. Herein, we describe a materials-based approach to augment the osteogenic potential of entrapped MSC spheroids by leveraging the mechanical properties of alginate hydrogels. Compared to spheroids entrapped in covalently crosslinked elastic alginate, calcium deposition of MSC spheroids was consistently increased in ionically crosslinked, viscoelastic hydrogels. We previously demonstrated that intraspheroidal presentation of Bone Morphogenetic Protein-2 (BMP-2) on hydroxyapatite (HA) nanoparticles resulted in more spatially uniform MSC osteodifferentiation, providing a method to internally influence spheroid phenotype. In these studies, we observed significant increases in calcium deposition by MSC spheroids loaded with BMP-2-HA in viscoelastic gels compared to soluble BMP-2, which was greater than spheroids entrapped in all elastic alginate gels. Upon implantation in critically sized calvarial bone defects, bone formation was greater in all animals treated with viscoelastic hydrogels. Increases in bone formation were evident in viscoelastic gels, regardless of the mode of presentation of BMP-2 (i.e., soluble delivery or HA nanoparticles). These studies demonstrate that the dynamic mechanical properties of viscoelastic alginate are an effective strategy to enhance the therapeutic potential of MSC spheroids for bone formation and repair.

Three-Dimensional Printed Stamps for the Fabrication of Patterned Microwells and High-Throughput Production of Homogeneous Cell Spheroids

Aggregation of cells into spheroids and organoids is a promising tool for regenerative medicine, cancer and cell biology, and drug discovery due to their recapitulation of the cell-cell and cell-matrix interactions found in vivo. Traditional approaches for the production of spheroids, such as the hanging drop method, are limited by the lack of reproducibility and the use of labor-intensive and time-consuming techniques. The need for high-throughput approaches allowing for the quick and reproducible formation of cell aggregates has driven the development of soft lithography techniques based on the patterning of microwells into nonadherent hydrogels. However, these methods are also limited by costly, labor-intensive, and multistep protocols that could impact the sterility of the process and efficiency of spheroid formation. In this study, we describe a one-step method for the fabrication of patterned nonadherent microwells into tissue culture plates using three-dimensional (3D) printed stamps and evaluate the production of cell spheroids of different sizes and cell sources. The generation of bone marrow-derived mesenchymal stromal cell and endothelial cell spheroids by the use of 3D printed stamps was superior in comparison with a widely used multistep mold technique, yielding spheroids of larger sizes and higher DNA content. The 3D stamps produced spheroids of more consistent diameter and DNA content when compared with other commercially available methods. These 3D printed stamps offer a tunable, simple, fast, and cost-effective approach for the production of reproducible spheroids and organoids for a wide range of applications.